A common visual motif often found in Hitchcock films is small blond leads. Hitchcock seems to favor small young blonde women to play the star. This is due to several reasons. First of all during the time that was the idea of beauty. Every women wished to be thin, blonde and beautiful and looks are always a big part of the movie industry. They were also chosen because they seem very naive which is why it is surprising that these women are so competent and able to have ulterior motives which is generally unexpected for the time.
In the film The Lodger, Daisy, the main character is at first portrayed as a simple model who is still living with her parents but it is later shown that she is very intelligent as she plays Joe, the policeman, into believing she reciprocated the feelings to some degree while really her attention was focused completely on the Lodger. This may have been due to the protection that being associated with Joe provided her with and it was only until her affections grew too strong for Jonathan (the Lodger) that she ended the ordeal with Joe.
In the film Blackmail, Alice, another young blond female lead is once again portrayed as foolish. She flirts like a school girl with the Artist and dresses in silly clothes for him. However when it comes down to it and she is uncomfortable she stabs the Artist to death. Many would believe such a young innocent gils would not be capable of committing such an act. This shows that Hitchcock really plays off of physical characteristics. Also having blonde hair and light skin reminds people of purity and that makes it easier to root for her character as being inherently good but being caught in a bad situation.
Therefore, in conclusion, Hitchcock uses young blonde girls to play his female leads because they are what was considered beautiful at the time, they appear innocent and surprise the audience with their intelligence considering the general population at the time believed women were generally less competent than men.